Torque limiter



GIOVERN/NG PROPELLER l l ff SPEED y e j; am y# 2,982,096 v roRQUE Lui-ITER Eugene J..Bevers`and Robert J. Wen'te, Indianapollavlnd.,

assignorsto General Motors Corporation, Detroit, Mich., a corporation of Delaware Filed May 28, 1957,1senNo, 660,495 s claims. (cito-M3938) Our invention relates to controls for gas turbine engines. It is particularly directed to a control `for -an engine1such the power output of the engine to' prevent mechanical overloading of the engine or reduction gear. c

By way of background to the description ofthe invention, various modes of control Ifor aircraft turbine propelhas proved most successful involves controllingthe 'power output ofthe engine by regulating the amount of fuel supplied to it and controlling the speed of the engine by a speed-governing propellerfThis sort of control isv emi ployed in normal takeoff, Hight, and 'landingl operation of the aircraft. The fuel controls which; are us'ed in`r` fu`chlV systems are of various types, but a vpreferred lty'pe lisa control which responds to certain controlling conditions such as rotational speedof the engine, temperature of air entering'the engine, and pressure of air entering' the en-V gine or leaving the compressor,`aud, from these controlling conditions,'determines a` rate of'fuel owto the engine which will maintain approximately aidesired turbine inlet: temperature. include means responsive vto turbine inlettemperature for more exact control of this condition.,` In other words, the lfuel supply under certain conditions is variedin direct ref spouse to turbine inlet temperature tomaint-ain this teinperature at a desired value or to prevent it from exceeding ,Q plant ymustratcd schematically comprises a .gas turbinc a safe limit. A o

Such a fuel control 'including direct responseto Iturbine inlet temperature is disclosed in application SerialNo. 496,094, tiled March 231955.(Paten't 2,938,340). Gas turbine fuelV controls also nearly always includeifanover speed governor to limit turbine speed'to a safe value.

Thus; withthepreviously knownand used fuel' con"- trols of which we are aware, the engine is efectively'pr'o`v tected againstovertemperature and voverspeed. The con-r trols include'rnechanisms which control theengine during start, acceleration and deceleration vto prevent" ilameout or compressor surge. The principal"d`etciency"of con-` trolssuch as those referred to lies inthe fact that they do not limit the power output of the engine, nor effectively conrol the power output. 'I'he essential mode of coni level, andthe power output of the engine may vary quite widely at any vgiven turbine temperature. 4The power output of the engine depends not onlyupon the turbine temperaturebut -very considerably upon the altitudefpres; i

sure, the temperature of the ambient air, and..the-'for ward lspeed of the aircraft. For a giventurbine inlet temperature, as ambient pressure increases, as forward speed' of the aircraft increases raising the ram pressue in the engine inlet, and as ambient tempeature decreases, T.

the power output will increase. Thusan `engine which is.` designed to deliver forpexample, `4,000' H.P.` under standard conditions at a normal flightV altitude of'25,0 00 feet may be capable of delivering some 6,000 H.P. at full speed forward night at vsea level with an ambientftempera'?. 1, u

ture ofi minus 65 degrees F. It not desirablevto pro- Such 4fuel controls frequently 2,982,096 Patented ,May 2, 1961 2 vide structural strength in the reduction gearing andV other parts of the engine ,to handle this possible 16,000 H.P. output.V Such excessive'power output might also be be yon'd the safe limitsv of the aircraft. This situation has previously been handled by providing an engine torque or poweroutpu't indicator, and relying upon the pilot to throttle back the engine if power exceeded the desired V V'I'hevpurpose of our invention is to provide a simple, effective and safe 'means to limit automatically the power output of vtheV engine to the desired maximum whenever the ambient conditions Vand flight situation are such that excessive .power might be deliveredV by the engine.

f ln its vpreferred embodiment, the invention is'based primarily upon the lfact that the power output of the engine ,l is Vapproximately adirect function of the Ifuel supplied to as lan Iaircraft turboprop engine which serves to'limit the engine. Put another way, the specific fuel consumption of the engine does not vary widely, so the power output for a given rate ofjfuel supply does not vary widely. Therefore, the engine delivers the Amaximum power Vif it is supplied an amount of fuelwhich is variable, ,but only ler power plants are known. The mode of control which f i to a limited extent. I

In view of this, it is possible tolimit the power output to a close approximation to the desired maximum by limiting the fuel flow ,to a predetermined maximum. This fact provide'sthe basis for the invention.A The invention, however, also s embodies means directly responsive to the torque output of the engine to trim or adjust the Amaximum fuel flow to hold the engine accurately at the power limit. A great advantage of our power limit control is that, if thetrimming controlrdirectly desponsive to torque'fails, the `flow-limiting device will `still limit the torque approximately `to lthe desired maximum. i' vThe nature'. of the invention andthe advantages thereof will be more clearly apparent to those skilled in thefart from the succeeding detailed description of the preferred embodiment of the invention and the accompanying drawing, which is a schematic diagnani of an engine and control system embodying the invention. `s

Referring rst yto the drawing, t-he turboprop power engine E having a power output shaft S coupledthrough reduction lgearing G to a speed-governing `propeller P. Fuel is supplied to the engine from a suitable source and line 10 byv a pump 11 driven by the engine through gearing V12, ,1-3. UThe Ifuel ows from the pump through a line;14to a main fuel control l16, the power (that is, essentially turbine temperature). setting of which may be varied by the vairc'sraft pilot through a control lever -17 and linkage 18. The main fuel control receives an input of f. engine speed through shaft 19, an input of engine inlet temperature from a temperature bulb 20 and'connecting line au input of engine inlet ram air pressure from a probe 23 through Ia line 2,4, and an` input of turbine inlet temperature; from thermocouples 26 connected to: the fuel Y, ze; controljthroughlnes 27' and 28. trol is to maintain the engine temperatureat a desired By mechanism immaterial to the present invention, the main yfuel control determines a fuel supply to the engine (inthe basisof the inputs of engine speed, inlet temperaturefai'idfpressure, and tu-rbine inlettemperature, The amount ofk fuely delivered by the control is` adapted to inaintainua 'desired turbine Ninlet temperature as set by the control lever V17; The excess of fuel delivered bythe pump -is returned to the pump inlet through a bypass' 31. 'Ihe metered fuel Adelivered from the fuel g control to the' engine is discharged through a line 32,

and,` after passing through the power limiting fuel corit'rol`of this invention, flows into the combustion section` of Athe ,engine` through a line 33 connected to fuel nozzleS34-f Itwwillbe undeistoodthat the engine and fuel control mechanism so `far described are previously known, and

' cenone that this invention is directed to the combination therewith of the fuel limiting control to be described;

The fuel limiting control also utilizes a power or torque measuring apparatus which may .be of any typt?, but, as illustrated, embodies a torque meter system of the type disclosed in U.S. Patent No. 2,766,617. It is illustrated only schematically herein. The torque meter includes a toothed wheel 41 onI the engine output shaft S and a toothed wheel -42 on a shaft 43 rigidly connected to the engine shaft. The torque exerted` by the engine twists shaft S and thus creates a phase displacement between the teeth of wheels 41 and 42. v Wheels 41 and 42 are armatures of induction generators and induce alternating current in pickup coils 44 and '46 connected to `a phase detector 47. The phase detector is an electronic device which develops la current proportional to the torque of shaft S. For the purposes of the control of this invention,

to pick off a voltage corresponding to the desired maximum torque. Slider 76 picks olf a voltage corresponding to the actual torque. The amplifier output is supplied through lines 87 and 88 to a torque motor 89 of known type, the movable armature 91 of which is connected by a link 92. to a rocker arm 93 the other end of which mounts the movable servo valve member 94. This valve member 94, as its position varies, varies the effective restriction of the outlet 96 from the servo chamber 97 in cylinder 71. Chamber 97 is supplied with `actuating fluid underpressure from any suitable source, which may be the outlet of pump 1.1. As illustrated, a servo supply line 98 runs from the pump outlet 14 through a pressure A regulating valve 99 and line 100, which has a restriction the current generated by the phase detector actuates a torque servo, which may be of known type, which rotates a shaft 49 proportionally to the current developed by the phase detector 47 and thus proportionally to the torque. The manner in which this measurement of torque is' employed will be described subsequently.

The metered fuel line 32 from the main fuel control is connected to the engine fuel line 33 through a venturi 51. The pressure loss in Vthe throat of the venturi is a measure'of the rate of fuel ilow and isl employed to operate a bypass valve 52 which limits the fuel ow to the engine. Valve 52 comprises a plunger or poppet 53 normally engaging a seat 54. A branch line 56 from the metered fuel `line 2.V leads to the seat 54 and the valve chamber 57 is connected through bypass line' S8 to the fuel return line 31. PlungerV 5.3 is slidably mounted in a guide 59 and is connected to a diaphragm 61 forming the wall between control chambers 62 and 63. Chamber 62 is connectedby pressure line 64 to the line 32 ahead of the venturi and chamber 63 is connected by pressure 101 in it, into chamber 97. The pressure in chamber 97 is a function of the relative restrictions of ow by restriction 101 and movable valve member 94.

As long as torque is below the preset limit value, the torque motor closes outlet `96 and piston 69 is held against stop 72 by the iiuid pressure. If the torque output reaches the limit value, the torque motor opens outlet 96, reducing the pressure in chamber 97, and the piston 69 movesV toward the right, reducing the force of spring 68. At

some point in the travel of the piston, the spring force will be suliiciently reduced to permit the diaphragm 61 to open valve S3, thus bypassing fuel and reducing the engine torque out-put. The travel of piston 69 is sufficient 4 that the lowest ow value which can cause the engine to reach the torque limit will be capable of opening valve 52 when the piston 69 reaches stop 73. The stop is adjustable for this purpose. A conduit i103 provides an outlet `from valve 94 and a drain for the cylinder to the bypass line.

line 66 to the throat of the venturi. The .pressure drop inthe venturi throat,rwhich is proportional to the square of ow, thus tendsv to lift thevalve plunger 53 from its seat to bypass some part of the metered fuel to thevpump' inlet. Plunger 53. is biased toward closed position by a' push rod 67 biased by a spring 68 which engages a mov-v able abutment constituted by 'a piston 69 reciprocably mounted in the cylinder 71 within which the spring 68 is mounted. Piston 69 has a limited range ofl travel between a first stop provided by a snap ring 72 in the cylinder wall and a second stop defined by a set screw 73 in the end of the cylinder.

As will be apparent, spring 68 tends to hold valve 52 closed in opposition to the opening force developed on diaphragm 61 by flow through the venturi. The force of spring 68 is determined by the position of piston 69. This force is variable over a rather narrow range s o that there is a rather narrow range of the critical value of fuel ow at which the pressure differential on the diaphragm will open valve S21. If the iiow rises above the critical value, valve 53 will open suiciently to bypass enough In order to stabilize the control of valve 52 by the torque meter, a feedback to amplifier 83 is provided. This is effected by a potentiometer 106 having a slider 107 mechanically coupled to the plunger 67 by any suitable connection indicated by the broken line 108. The potentiometer 106 is connected to ground at one end and lto the amplifier 83 by a line 109. The slider is connected to the amplifier by a line 111. Movement of the plunger thus varies the potential across lines 109 and 111, providing a feedback to the amplifier which may be employed fuel into line 58 to reduce the ow to the engine to the v value at which the force of spring 68 balances the flow inducedV pressure differential on diaphragm 61.y Y

The position of piston 69 is controlledby thev torque servo 48. The output shaft 49 of this servo drives movable contact` 76 of potentiometer 77 through gearing 78. The position of the movablecontact is thus directly re# sponsive to the actual torque delivered by the engine. Potentiometer 77 and a second potentiometer 79 areconnected in a parallel circuit between' a currentsource 81 and ground. The movable contact 76` is connectedby line 82 to a motor control amplifier 83. This amplifier also receives a voltage input from manually settable slider 84 yof potentiometer 79 through line l86. The ampliiier provides an output which is responsive to thediiference of potential of the wires 82 and 86. Slider -84 may be set in known manner to stabilize the operation of the torque control of the bypass valve 52.

The operation of the control may be illustrated by an example: Let us suppose that the engine is to be limited to,4,000 H.P. and that it is capable of generating 6,000

-' H.P. at full speed, low altitude flight on a very cold day.

We may also assume that 4,000 H.P. will be developed at a fuel rate of 1,600 to 2,000 lbs. per hour, depending upon ambient and ight conditions and the condition of the engine; that 4,000 H.P. will always be generated if the fuel consumption is over 2,000 lbs. per hour and cannot be generated if the fuel consumption is less than 1,600 lbs. per hour. Also, it is preferable to permit some overload rather than to risk` cutting power much below the maximum limit. We may assume that 1,800 lbs. per hour willalways generate at least 3,600 H.P. and will not generate over about 4,400 H.P. The bypass valve begins to open at a ow of 1,800 lbs. per hour when the piston 69 is against stop 73 and will open sufliciently to bypass a third of the fuel delivered by the main fuel control when the flow reaches 2,000 lbs. per hour and the piston is against stop 72.

If the plane is fiyingat full power setting at high altitudes, the fuel consumption will be low. If we assume that the plane descends at full power or possibly encounters a cold front at moderate altitudes, the main fuel control will deliver more fuel than 2,000 lbs. per hour, since the engine will accept this without excessive turbine temperature. As long as the torque is below that corresponding to 4,000 H.P. the piston will be against stop 72, putting the maximum loading on the valve 52. If the 'fuel flott)l reaches, 2,000flbs'. per the bypa/s'sivalvc 4will beginv to open.V Howeven-normally the 'torque' Awillreach the limit'value before t'hefuelow reaches 2,000 lbs. per hour and,as soon as it does, the servo system will open valve 94 to allow piston69tomoveto'the right. It will continue to move to the right as longv as the torque is excessive until the loading of spring V68 yis 'reduced to the point at which valve 62 will opentobypass fuel and hold the torque tothe 4,0'00*H;Pf level.; J 1

This leads to discussion of an important advantage of the invention. Ifthe torque meter and the servo'system, includingth'eamplier and valve 9 4 work properly,'the torque is h`eld`y-to the exactl desiredV limit, because 'spring 68 can be loaded to obtain this result over the entire range of speciiicfuel consumption at 4,000 H.P. However, even Vif these fail,"the fuel'will be limited to a maximum value of from 11,800 to 2,000 lbsiper hour, depending uponthe position of piston 69, Thus, under the most unfavorable,V conditicmsA of failure of theservo` ,system and specific,` fuel consumption, thetorque limiter cannot reduce the-fuel to less than 90% of that corresponding to the limitingftorqueA value andwill never let fuel -tlow to the engine go to more Athan 110% .of that developing the maximum torque limit. Thus, there is a very simple con-V trol involving only the'vent-uri, the diaphragm, the bypass valve, and thespring .which will, under any circumstances, limit ,powerwat someipoint reasonably-close to the desired limit. If the electronically controlled Yservo system is operating properly, the exact limit will be maintained. If the electronic system were relied upon entirely andthe force of spring 68 were not limited by the stops 72 and 73, failure of the servo system might result in bypassing a third of the fuel supplied to the engine. Such an occurrence on takeoff, for example, might be disastrous.

It will be noted that the foregoing discussion has referred to torque and power as directly related values, which they are in a system of this sort if the propeller is driven at constant speed. However, the control, as described and as preferred, responds to torque. Even if the propeller speed varies .to some extent, the torque is the lbest measure of overload on the reduction gear and `other mechanisms.

The detailed description of the preferred embodiment of the invention for the purpose of explaining the principles thereof is not to be considered as limiting the invention, as many modifications may be made within the scope of the invention by the exercise of skill in the art.

We claim:

l. The combination of a gas turbine engine having a power output shaft and having the characteristic that a redetermined torque output is developed by the engine in a normal operating regime thereof Ain consequence of fuel flow to the engine approximating a predetermined value, means for supplying fuel to the engine including a main fuel control regulating the fuel supply to the engine and normally determining the power output thereof, a fuel line connecting the main fuel control to the engine conducting all the fuel to the engine, a flowmeter in the fuel line, and means controlled by the owmeter connected to the fuel line for limiting fuel ow to the engine to the said predetermined value.

2. 'I'he combination ofa gas turbine engine having a power output shaft and having the characteristic that a predetermined torque output is developed by the engine in a normal operating regime thereof in consequence of fuel flow to the engine approximating a predetermined value, means for supplying fuel to the engine including a main fuel control regulating the fuel supply to the engine and normally determing the power output thereof, a fuel line connecting the main fuel control to the engine conducting all the fuel to the engine, a owmeter in the fuel line, and means including a bypass from the fuel line upstream of the llowmeter anda valve in the bypass controllcd by the owr'neten 'for' limiting fuel dw to thereugine tothe said predetermined value.- l '1 'l y 3.''A"c ontrol system fora gaSturbine 'engine having aj poweroutputsha'ftj and having the characteristic that a predetermined torque output is developed by theengine in a normal operating regime thereof in consequencefof fuel `flow ato the engine approximating a predetermined value, the system comprising-in combinatiomme'ans for supplying fuelto the engine including-a main fuel control regulatingthe fuel supply 'to theA engine and normally determining the power output thereof, a fuel line coni necting the main 'fuel control tothe v'engine conducting alll'of the fuelrv to the engine, a flowmeter inthe fuel line, means `including la bypass fromthe fue] line upstream of the owmeter and avalve in the bypass controlledby the flowmeter for limiting fuel ow to the engine to the said predetermined value, means responsive .to the'torqu output of the engine, and means controlled by the torqueresponsive means connected to 'the valveto adjust the valve position so as to modify f uel llow to` the engine to lthe vvalue effective to generate the said predetermined torque output. .A i

" 4. VAfuel control system -for a gas turbine engine having the characteristic that a predetermined maximum torque is developedv by the `engine in anormal operating regime thereof in consequence of vfuel flow to Ithe engine within a 'predetermined'range of values, the system corn-A prising, `in combination, means for supplying fuel,'a main fuel control for metering the fuel and normally determining the power output of the engine, a fuel line connecting the main fuel control to the engine, a venturi in the fuel line, a bypass conduit from the fuel line ahead of the venturi, a valve in the bypass conduit, means responsive to pressure drop in the venturi biasing the valve toward open position, and a spring biasing the valve toward closed position, the spring being sufficient to hold the valve closed until the fuel flow reaches a value Within the `said predetermined range.

5. A fuel control system Ifor a gas turbine engine havlng the characteristic that a predetermined maximum torque 1s developed by the engine in a normal operating regime thereof in consequency of fuel flow to the engine within a predetermined range -of values, the system comprising, in combination, means for supplying fuel, a main fuel control for metering the fuel,l a fuel line connecting the main fuel control to the engine, a venturi in the fuel hne, a bypass conduit from the fuel line ahead of the venturi, a valve in the bypass conduit, means responsive to pressure drop in the venturi biasing the valve toward open position, a spring biasing the valve toward closed position, an abutment engaged by the spring movable between limits providing a spring loading range corresponding to the said predetermined range of fuel flows within which the predetermined maximum torque of the engine is developed, and means responsive to torque output of the engine coupled to the abutment so as to vary the loading of the spring in such sense as to limit torque output to a predetermined value.

6. A fuel control system for a gas turbine engine having the characteristic ythat a predetermined maximum torque is developed by the engine in a normal operating regime .thereof in consequence of fuel flow to the engine within a predetermined range of values, the system comprising, in combination, means Kfor supplying fuel, a main fuel control for metering the fuel, a fuel line connecting the main fuel control to the engine, a venturi in the fuel line, a bypass conduit from the fuel line ahead of the venturi, a valve in the lbypass conduit, means responsive to pressure drop in the venturi biasing the valve toward open position, a spring biasing the valve toward closed position, an abutment engaged by the spring movable between limits, the limits of movement of the abutment providing a spring loading range elfective to cause opening of the valve within the range of venturi pressure drop resulting from fuel ows within the said predetermined 7 range of values, means. responsive to the. torque output of the engine, and means controlled by the torque-responsive means connected to the abutment toL move the abutment so as to modify fuelv flow tothe engine to the value effective to generate the said predetermined maximum torque output. ,f

7. A fuel control system for a gas turbine engine having the characteristic that a predetermined maximum torque is developed bythe engine in a normal operating regime thereof in consequence of fuel ow to the engine within a predetermined range of values, the system comprising, in combination, means for supplying fuel, a main fuel control for metering the fuel, a fuel 'line connecting the main fuel control to the engine, a venturi in the fuel line, a bypass conduitl from the fuel line ahead of the venturi, a valve in the bypass conduit, means responsive to pressure drop in the, venturi biasing the valve toward open position, a spring biasing the valve toward closed position, and an abutment engaged by the spring movable between limits, the limits of movement of the abutment providing a spring loading range effective to cause opening of the valve within the range of venturi. pressure drop resulting from'fuel flows within the said predetermined range of values,

8. The combination of a gas turbine engine having a power output shaft, and having the characteristic that a predetermined torque output is developed by the engine in a normal operating regime, thereof in consequence of fuel flow to the engine approximating a predetermined value, means for supplying fuel` tothe engine including a main fuelcontrol regulating the fuelv supply to the engine and normally determining the power output thereof, a-fuel line connecting the main fuel control to the engine conducting all the fuel Ito the engine, a owmeter in the fuel line, means including a valve controlled by the flowme'ter connected to .the fuel line for limiting fuel ow to the engine to. the said predetermined value, means responsive to the torque output of the engine, and means controlled by the torque-responsive means connected to the valve to adjust the valve position so as to modify fuel flow to the engine to the value effective to generate the said predetermined torque output.

References Cited in the file of this patent UNITED STATES PATENTS 2,303,998 Holley Dec. 1, 1942 2,404,428. Bradbury July 23, 1946 2,560,118 Malone et al. July 10, 1951 2,702,560 Bobier Feb. 22, 1955 2,708,826 Torrell y May 24, 1955 2,714,803 Abild Aug. 9, 1955 FOREIGN PATENTS 617,729 Great Britain Feb. 10, 1949 769,103 Great'Britain Feb. 27, 1957 

